4.2 Study procedures All blood samples (3mL each) will be taken from an arterial or venous catheter and collected in additive-free blood tubes. After blood clotting, they will be centrifuged and the serum will be frozen (-80°C) until analysis. Total concentration and free concentration will be determined by a validated High Pressure Liquid chromatography-Ultra Violet detection (HPLC-UV) method.12,17